The brain is the most complex organ in the human body. For a healthy person with a brain acting naturally, the brain's multiple parts work together to sustain and enhance life.
Drug use disrupts the way the brain naturally operates, altering the way the brain communicates with the different parts of itself and the rest of your body. Over time, these effects can radically alter the way your brain works.
So while it may seem that a drug has no effect after the sense of "high" fades, in fact, consistent drug misuse is changing the brain in unseen, and harmful ways.
How the brain communicates
The human brain has multiple parts to it, different "sections" that must work together in order for automatic bodily functions, what we perceive from the outside world through our senses, and how we think and process information. These different brain sections are connected by what are called neurotransmitters, or chemical signals that travel from one neuron (or brain cell) to the other.
One of these neurotransmitters is called dopamine, one of the central parts of the brain that creates feelings of pleasure. When you have a fun interaction with a friend, hear some good music, do a fun physical activity, or see some beautiful in art or in nature, the neurons in the your brain are receiving dopamine, causing pleasurable and happy feelings.
Since the dopamine causes you feel good doing these activities, it can be a powerful motivator to keep doing those things, creating what is called a "reward cycle." That is why eating a delicious meal, and engaging in physical exercise, things necessary for health, produce high amounts of dopamine.
How drugs affect the brain to seem pleasurable at first...
The intense "high" feelings of drug use come about from overwhelming the brain with dopamine or other neurotransmitters that produce pleasurable feelings. An abused drug like cocaine can immediately flood the brain with dopamine, sometimes as much as 10 times what could be produced "naturally" through eating and sex.
This is the central reason why drugs of abuse can become so addictive, because they produce such a powerful reward unmatched by anything else, and become a powerful motivator for using again. After a pattern of drug use has set in, the brain's pleasure processing center will no longer respond to the comparatively minimal amount of dopamine produced by pleasurable activities, and will instead become singularly focused on a craving for the substance.
...But damages things over time
Even the overwhelming surge of dopamine from drug use will not last forever. Being bombarded with such a high level of dopamine will inhibit the neuron's ability to process it at all, dampening its impact and producing depressed, lifeless feelings, and an inability to feel any pleasure at all.
The addict then has to take a substance simply to feel "normal," and then take it at ever increasing and more risky levels in order to feel that "high" again. This need for an increased dosage is referred to as "tolerance."
Furthermore, the decreased ability of the neuron's to receive neurotransmitters can occur for more then dopamine. Often, addicts' brains are also unable to receive glutamate, a central neurotransmitter affecting our ability to learn and process information.
The brain of a long-term addict may thus be working slower, or develop other strong cravings or compulsions unrelated to the addiction itself. High levels of drug use can even kill neurons, decreasing brain cells and making it that much harder for the brain to process information.
The brain is an incredibly complex organ, and there is a need for more study to know exactly how the brain is affected by various abused substances. However, it is undeniable that the pattern of addiction is itself very harmful to the brain's processing.